1. Field of the Invention
The present invention relates to storage formats for image data and in particular, to storage formats that allow efficient representation of images with extended color gamuts, while maintaining compatibility with legacy hardware and software that have limited color gamuts.
2. Description of the Related Art
The gamut of an imaging system refers to the range of colors that can be produced or represented by that system. Many imaging devices, such as cameras and printers, can capture and/or produce colors that are outside the gamut of a standard cathode ray tube (“CRT”), which stores data in an RGB color space with 8 color bits for each of a red, green and blue channel. The mismatch between the RGB color gamut and the color gamuts of other output devices and image sources represents a serious limitation on the usefulness of the RGB color space. Nevertheless, for many applications it is convenient to store, display and manipulate a digital image in a particular storage color space that is well-suited for the work flow associated with that application. Additionally, many software applications that are available to manipulate images on a computer are designed to work with images in an RGB color space.
Because there will generally be colors in an extended color gamut digital image that can not be represented in an RGB color space, use of this color space will come at the expense of applications that can utilize the extended color gamut information that may have existed in an input image. Yet 8 bit per channel RGB images have long been a de facto standard. With the introduction of IEC's sRGB standard, they have become a de jure standard as well.
To overcome this problem, proposals have been made for creating an RGB color space with extended gamut. Most of these proposals require the use of more than 8 bits per color channel. For example, the IIIA's esRGB proposal uses the same three primaries and maintains compatibility with sRGB within the 8 bit range, but then extends the gamut by allowing code values less than zero and greater than one. Other proposals, like Kodak's RIMM and ROMM RGB, move the three primaries farther from white point. Kodak's proposal supports 8, 10, and 12 bits per color channel. The 8 bit encoding is not compatible with sRGB, and achieves a wider gamut at the cost of noticeable quantization levels within the image. The IEC'S scRGB requires 16 bits per color channel and uses a linear luminance encoding rather than a linear brightness encoding.
The problem with these proposals is that images encoded using them cannot be displayed properly on legacy equipment. Images that use more than 8 bits per channel will be completely unrecognizable. Images encoded using a signed representation will be almost as bad. Values in the images near zero will appear at mid-level brightness; negative values will shift their relationship to the device primaries; that is, those that were farther from device white than the device primaries will suddenly become closer to device white. Therefore these proposals fail to develop an image representation that allows representation of colors that are outside a CRT monitor gamut, but are still compatible with legacy hardware and software.
More recently, patents assigned to Kodak, such as U.S. Pat. No. 6,282,313 to McCarthy et al entitled “Using a Set of Residual Images to Represent an Extended Color Gamut Digital Image”, describe a method for representing a digital image having color values with an extended color gamut by adjusting the color values of an extended gamut image to form a limited color gamut digital image and determining a set of residual images to represent the difference between the extended color gamut digital image and the limited color gamut digital image. The residual images and the limited color gamut digital image can then be used to form a reconstructed extended color gamut digital image.
These methods suffer from lack of accuracy in that the calculated residual image is subject to round off errors in performing subtractions. They are also inefficient due to the large storage space that is required for storage of both the limited color gamut digital image and the residual image.